This invention relates to the construction of semiconductor devices and other layered devices by photolithography including steppers for displacing a semiconductor wafer relative to a mask or a reticle and, more particularly, to the use of latent images of alignment marks formed in a photoresist layer for increased precision in the locating of a wafer relative to a mask.
Numerous forms of semiconductor devices including integrated circuits, by way of example, are constructed by building up layers of material on a substrate, etching away portions of the material, followed by steps of deposition of other substances which may include dopants. The locations of the various materials are precisely controlled to insure an accurate formation of the various semiconductor structures. Typically, layers of photoresist are deposited and exposed by photolithographic apparatus including masks to delineate the forms, in two dimensions, of the desired structural elements. In the case of construction of numerous chips from a single wafer, a mask used in the construction of any one chip is also used for constructing the other chips. This is accomplished by stepping the wafer under an optical system including the mask for exposure of the individual wafer regions, or dies, of the respective chips. At each die, photoresist is exposed, typically with ultraviolet light by the optical system to form a latent image of the mask in a layer of photoresist. The wafer is then removed from the stepper, and developer is applied to the photoresist to develop the image. Etches selective to the development of exposed photoresist remove portions of the photoresist to prepare the wafer for the next stage of material deposition or other types of etching processes. At a later stage in the manufacture the wafer is returned to the stepper for exposure of the dies to another mask.
It is noted that the foregoing use of photolithographic apparatus in the printing of successive circuit patterns on the surface of a wafer can also be employed in the manufacture of reticles or masks is in the manufacture of semiconductor devices. Generally speaking, the use of the term mask is understood to include the term reticle or E-beam or maskless direct write control pattern for the purposes of the present invention.
To ensure accurate alignment of the wafer relative to the optical system, it is the practice in the initial stage of manufacture to produce tool registration aids on the wafer substrate in the marginal region surrounding each die, the registration aid being outside the product area of a die. A tool registration aid may be formed as a depression in the substrate surface, for example, wherein the aid is formed by an etch. A primary verification aid or mark may also be formed on the substrate. During subsequent steps in the manufacturing process, secondary verification marks located in the marginal region of a mask are projected onto a layer of photoresist and developed to allow alignment of the primary and the secondary verification marks.
The foregoing procedure presents a problem in that the verification marks imprinted in the photoresist must be observed in a manner which does not detract from the use of the photoresist in the process of imaging the details of an integrated circuit or other subject matter of the semiconductor device. Under present practice in which the verification marks imprinted on the photoresist are not usable until after the photoresist has been developed, the wafer is removed from the stepper, processed with a developing liquid, and then examined under a microscope to determine the degree of alignment between verification marks and registration marks. This wafer is known as a send ahead wafer, and must be reprocessed in the event that the alignment is poor. The stepper is then adjusted to correct the alignment, after which subsequent wafers can be processed by the stepper. The current practice is particularly inconvenient where relative few wafers are to be processed, and wherein frequent reticle changes may be required by a need to place several product patterns on a single wafer. The send-ahead wafer is a randomly selected product substrate from a batch. Alternatively, a monitor substrate which does not have a product being developed thereon may be employed to check alignment. In either case, the additional manufacturing steps represent a significant inconvenience.